Elton, who has been called the father of ecology, published a 1958 book on invasions of exotic species in which he proposed his idea, which is known as the Diversity-Resistance Hypothesis. Since then, many scientists have explored the hypothesis, said Theodore Kennedy, a graduate student in ecology and lead author of the Nature paper.
"Researchers studying sedentary marine invertebrates have found that diverse communities make more complete use of the limited space available to them, and this leaves invaders with no place to settle," he said. "But the mechanism by which diverse plant communities keep out newcomers was previously unknown."
Kennedy and his colleagues asked whether land supporting relatively large numbers of prairie plant species would do a better job of excluding plants of Eurasian origin. They studied 147 square plots of land, each three meters on a side, at the university's Cedar Creek Natural History Area near the Twin Cities. Each plot had been planted with either 1, 2, 4, 6, 8, 12 or 24 grassland plant species, and the researchers had already noticed more weedy biomass in the less diverse plots. For two years they ceased weeding and examined relations between Eurasian weeds and "resident" species (those that had been purposely planted) within small (40 centimeters x 125 centimeters) sections of each plot.
The team found that the total number of invading weeds, the number of extremely large individual weeds, and the total amount of ground covered by weeds all decreased as the number of resident plant species increased.
The researchers also examined the placements of invader and resident species. For each invading weed, they counted all the resident plants in a 10-centimeter radius "neighborhood." They also computed a "crowding index" that took into account the number, nearness and size of the resident plants. In more diverse plots, invader weeds faced higher numbers of resident plant "neighbors" and a higher crowding index. The greater the crowding in the neighborhood, the smaller the invader weed size.
To further examine the role of spatial crowding, the team chose 100 point locations at random in each plot, counted the resident plants in the 10-centimeter neighborhoods of the points and figured the average for each plot. They found that invader weeds tended to grow in neighborhoods that were less crowded than the average for the plot.
The results suggest a mechanism of invader repulsion similar to what goes on with marine invertebrates, Kennedy said.
"Here, it appears we also see a more complete use of space," he said. "We found the neighborhoods of weeds were more crowded in the plots with high species diversity. We think that in addition to a lack of space, there are fewer other resources available to invaders in crowded neighborhoods. That is, when you pack more plants into a small area, you'll have more complete utilization of nitrogen, phosphorus, water, light, and other resources that will thereby limit the success of invaders."
The findings imply that losses of biodiversity could also mean loss of local resistance to invasion by species that, thanks to human transport, are no longer kept out by geographical or other barriers, the researchers said. Therefore, restoration and revegetation efforts would likely benefit from establishing as diverse a plant community as possible. Diverse communities should require less maintenance and monitoring because, the researchers write, "they are generally very effective at excluding undesirable invaders."
Kennedy's colleagues were Shahid Naeem and Katherine Howe of the University of Washington, Johannes Knops of the University of Nebraska and David Tilman and Peter Reich of the University of Minnesota. The work was supported by the Andrew Mellon Foundation, the McKnight Foundation, the University of Minnesota, NASA and the National Science Foundation.
Theodore Kennedy, (760) 873-4356 or (510) 435-5277 (in California June 2-8), firstname.lastname@example.org
Deane Morrison, University News Service, (612) 624-2346, email@example.com (call for an e-picture of Kennedy)